The principle of recording devices of an ink-jet recording type consists in that liquid or melted solid ink is jetted through nozzles, slits, porous films or the like to make recording on paper, cloth or film sheets. As methods for jetting ink, various types of systems have been proposed such as the so-called charge control system in which ink is extruded by making use of electrostatic attraction, the so-called drop-on-demand system (pressure pulse system) in which ink is jetted by making use of vibrational pressure of a piezo element, and the so-called thermal ink-jet system in which ink is jetted by making use of pressure resulting from foams formed and developed by heightened heat. These systems make it possible to obtain extremely highly precise images.
As the ink used for such ink-jet recording systems solutions or dispersions are known and employed in which various water-soluble dyes or pigments are dissolved in water or liquid media comprising water-soluble organic solvents.
Of various requirements for an ink composition for ink jet recording, the most demanded one is liquid stability during use. That is, when recording is suspended or when recording is not effected for a long time, an ink composition should not cause clogging of nozzles or orifices of a recording apparatus or not to produce sediment. In a thermal ink jet system, in particular, foreign matter is apt to be deposited on the surface of a heater, which is for forming and growing bubbles in an ink, with changes in temperature. The foreign matter deposit on the head of the heater is called kogation. In long-term recording operation, kogation is released to cause a change in ink output, leading to variation of image quality. Conventional ink compositions contain several additives necessary for satisfying various conditions, such as ink jet conditions, stability against long-term storage, image clearness and density, surface tension, electrical properties, and the like as well as various impurities originated in the dye or pigment. Therefore, the ink compositions tend to cause clogging of nozzles or orifices, kogation on the head of a heater, and, when left out of use for a long period of time, produce sediment at nozzles or orifices.
In order to solve these problems, various proposals have been made. For example, JP-B-3-48950 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses adjustment of the iron content of an ink composition to 4 ppm or less. JP-B-3-48951 teaches adjusting the total content of iron and silicon in an ink composition to 9 ppm or less. JP-B-3-48952 proposes adjusting the magnesium content of an ink composition to 4 ppm or less. JP-B-3-48953 suggests adjusting the total content of calcium, magnesium, manganese, iron, aluminum, and silicon in an ink composition to 20 ppm or less. Further, JP-B-3-48954 proposes adjusting the content of lyophobic colloid in an ink composition to 13 ppm or less. However, none of these ink compositions proposed succeeded in perfectly solving the above-mentioned problems.
In particular, staying of foams formed inside ink passages in ink-jet recording devices results in prevention of the uniform streams of ink, and causes defects in images such as blank areas on images, fluctuations in dot diameter, and the disturbed shape of dots. Hence, a great number of methods for controlling the formation of foams in ink have hitherto been proposed.
For example, in recording liquids containing surfactants, JP-A-61-250077 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses to restrict the foaming property of the surfactants according to the Ross-Miles method to 150 mm or less in foam height of 0.1% by weight aqueous solutions after 5 minutes. JP-A-63-139963 proposes to adjust foam stability after 5 minutes to 0 mm. Further, JP-A-2-151674 offers the use of ink of 10 to 200 mm in formability and 10 to 200 mm in foam stability, and JP-A-4-239067 proposes to restrict the HLB value of surfactants to 10 to 20 to prevent defects in images due to foaming.
However, the control of foaming properties in conventional methods are effective only when ink passages in devices have smooth surfaces formed of glass or silicones. Passages in actual devices are not sufficiently satisfactory because they have complex shapes and are formed of two or more kinds of materials such as plastics and rubber materials. That is, the Ross-Miles method denoted in JP-A-61-250077, JP-A-63-139963 and JP-A-2-151674 determines the behavior of foams on smooth glass surfaces, and is therefore effective for controlling foaming on smooth glass surfaces and silicone surfaces having similar surface properties to those of glass. However, this method is not satisfactory to surfaces of complex shapes constituting the inside of the passages or to portions composed of plastics or rubber materials. Further, the method proposed in JP-A-4-239067 is also not sufficiently satisfactory.